1. Field of the Invention
This invention relates to a semiconductor light emitting device especially suitable for use as a semiconductor light emitting device using II-VI compound semiconductors.
2. Description of the Related Art
Recently, semiconductor lasers used for record/reproduce of high-density optical discs or magneto-optical discs to emit blue to green light, and light emitting diodes used in large-scale displays or signal lamps to emit blue to green light, are under active researches and developments.
The most hopeful materials for manufacturing these semiconductor light emitting devices for emission of blue to green light are II-VI compound semiconductors combining a group II element such as zinc (Zn), cadmium (Cd), magnesium (Mg), mercury (Hg) or beryllium (Be) and a group VI element such as sulfur (S), selenium (Se), tellurium (Te), or oxygen (O). Semiconductor light emitting devices using these II-VI compound semiconductors are currently under improvements of device characteristics and the lifetime.
Multiplication of dislocations derived from stacking defects in the active layer was pointed out as an important factor of shortening the lifetime of a semiconductor light emitting device using II-VI compound semiconductors. In order to decrease such stacking defects, various approaches were made, such as growing a GaAs buffer layer on a GaAs substrate prior to growth of II-VI compound semiconductors, or optimizing the initial sequence of growing II-VI compound semiconductors. As a result, the lifetime of semiconductor light emitting devices using II-VI compound semiconductors was elongated to over 100 hours at the room temperature. Thereafter, along with the decrease in stacking defects, point defects in active layers have been pointed out as a factor of deterioration of devices. In order to prevent multiplication of point defects, although it is important to establish a growth condition which decreases point defects themselves, it is also necessary to stabilize the electronic state of point defects and to decrease their mobility so as to prevent their coupling.
Moreover, in semiconductor light emitting devices including a ZnCdSe active layer, it has been confirmed that a certain emission wavelength shifts to a shorter wavelength probably because of diffusion of Cd upon a supply of an electric power and that the p-type carrier concentration decreases by annealing. Therefore, device characteristics might be changed by diffusion of component atoms of a crystal or impurities. However, it is not easy to artificially control these diffusion phenomena.
On the other hand, it has been reported that, in devices using a ZnCdSe quantum well layer doped with nitrogen as a p-type impurity, annealing invites serious diffusion of Cd. It is explained as resulting from an unstable electronic state of vacancies of group II atoms (hereinafter called "group II vacancies") existing in the crystal and from an increase in mobility. That is, since group II vacancies intrinsically stabilize by getting free electrons as acceptors, when electrons become less in the crystal as a result of p-type doping, the electronic state of group II vacancies becomes unstable and liable to move. Since group II atoms, Cd, diffuses via movements of vacancies, Cd also becomes liable to move along with the increase in mobility of the group II vacancies. In this manner, diffusion of component atoms of the active layer is closely related to the doping method employed, it is desirable to optimize it so as to minimize deterioration of the active layer.
Semiconductor light emitting devices having a separate confinement heterostructure (SCH) are characterized in effective confinement of light because of the structure where light cannot easily exude from the active layer into the cladding layer having a lower refractive index. However, since it is impossible to completely confine light, there is the possibility that light exuding into the cladding layer excites the energy state of originally unstable atoms or defects and they diffuse toward the active layer. Especially, chlorine (Cl) and nitrogen (N), which are impurities of II-VI compound semiconductors have smaller lattice constants as compared with component atoms such as Zn and Se, are considered to be weak in coupling force between atoms, and are apt to get into unstable energy states. Actually, in a semiconductor laser using II-VI compound semiconductors, it was confirmed that the carrier concentration of a p-type cladding layer decreases under a supply of electric power, and it causes an increase in threshold current value. A possible reason thereof may be that N atoms in the p-type cladding layer are excited by light, get off from lattice sites of group VI atoms, and inactivated or that Cl atoms in the n-type cladding layer are exited similarly, propagate and reach the p-type cladding layer, and compensate acceptors.
In an experiment made by the Inventors, when a semiconductor light emitting device was electrically powered, Cl atoms in the n-type cladding layer diffused and moved the position of the p-n junction. More specifically, a semiconductor light emitting device as shown in FIG. 1 was prepared, and the position of the p-n junction was measured before and after electric power supply to the semiconductor light emitting device by an electron beam induced current (EBIC) method. As a result, before being electrically powered, the p-n junction entirely appeared in the active layer as shown in FIG. 2, but after being electrically powered, a part of the p-n junction in the stripe region moved toward the p-side as shown in FIG. 3. In FIGS. 2 and 3, the EBIC signal is shown by the hatched region. Presumably, such a movement in position of the p-n junction decreases the injection efficiency of carriers into the active layer, increases the threshold current value, and decreases the lifetime of the device.
Under the situation, it has been difficult heretofore to realize a semiconductor light emitting device with good characteristics, high reliability and long lifetime.